U.S. patent number 4,603,722 [Application Number 06/655,288] was granted by the patent office on 1986-08-05 for tire tread comprised of branched styrene-butadiene copolymers.
This patent grant is currently assigned to Bridgestone Tire Company, Japan Synthetic Rubber Co., Ltd.. Invention is credited to Tatsuo Fujimaki, Noboru Oshima, Isamu Shimizu, Seisuke Tomita, Yoshito Yoshimura.
United States Patent |
4,603,722 |
Oshima , et al. |
August 5, 1986 |
Tire tread comprised of branched styrene-butadiene copolymers
Abstract
A branched styrene-butadiene copolymer and a pneumatic tire
using the same as a tread base rubber are disclosed. This copolymer
is produced by coupling an active styrene-butadiene copolymer
anion, which is obtained by polymerizing styrene and butadiene in a
hydrocarbon solvent in the presence of ether or a tertiary amine
and an initiator of an organolithium compound, with a tin halide
compound. In the copolymer, the ratio of branched copolymer
connected with tin-butadienyl bond is at least 20% by weight, and
the content of bound styrene is not less than 3% by weight but less
than 25% by weight, and the content of vinyl bond in butadiene
portion is not less than 30% but less than 50%.
Inventors: |
Oshima; Noboru (Suzuka,
JP), Shimizu; Isamu (Kameyama, JP),
Yoshimura; Yoshito (Yokkaichi, JP), Fujimaki;
Tatsuo (Higashimurayama, JP), Tomita; Seisuke
(Tokorozawa, JP) |
Assignee: |
Bridgestone Tire Company
(Tokyo, JP)
Japan Synthetic Rubber Co., Ltd. (Tokyo, JP)
|
Family
ID: |
26382693 |
Appl.
No.: |
06/655,288 |
Filed: |
September 28, 1984 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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473394 |
Mar 8, 1983 |
4526934 |
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Foreign Application Priority Data
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Mar 19, 1982 [JP] |
|
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57-42955 |
Mar 25, 1982 [JP] |
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57-47946 |
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Current U.S.
Class: |
152/209.5;
525/332.3 |
Current CPC
Class: |
C08C
19/44 (20130101); B60C 1/0016 (20130101) |
Current International
Class: |
B60C
1/00 (20060101); C08C 19/44 (20060101); C08C
19/00 (20060101); C08F 008/18 () |
Field of
Search: |
;152/29R,374
;525/99,315,371,332.9,332.3 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Ball; Michael
Attorney, Agent or Firm: Sughrue, Mion, Zinn, Macpeak, and
Seas
Parent Case Text
This is a division of application Ser. No. 473,394, filed 3/8/83,
now U.S. Pat. No. 4,526,934.
Claims
What is claimed is:
1. A pneumatic tire comprising a toroidal carcass reinforcement,
and a tread portion superimposed about a crown region of said
carcass and having a cap/base structure of tread base rubber and
tread cap rubber, the improvement wherein said tread base rubber is
a rubber composition containing (1) at least 20 parts by weight of
branched styrene-butadiene copolymer rubber based on 100 parts by
weight of the total rubber content, wherein said branched
styrene-butadiene copolymer rubber has a bound styrene content of
3-15% by weight and a vinyl bond content in the butadiene portion
of not less than 30% but less than 50%, and is produced by coupling
a styrene-butadiene copolymer anion with a tin halide compound and
(2) not less than 30 parts by weight of at least one of natural
rubber and polyisopropene rubber based on 100 parts by weight of
the total rubber content.
Description
The present invention relates to branched styrene-butadiene
copolymers and pneumatic tires using the same. More particularly,
it relates to a branched styrene-butadiene copolymer having a low
hysteresis loss and improved fracture properties and a pneumatic
tire having improved high-speed durability and rolling resistance
by using the branched styrene-butadiene copolymer as a tread base
rubber in tires comprising a tread portion of a cap/base
structure.
Lately, it has been strongly demanded that rubber materials having
a low hysteresis loss be employed as a rubber for tires to achieve
reduction of fuel consumption in automobiles. As the rubber
material having a low hysteresis loss, there have generally been
used rubber blends of natural rubber or synthetic cis-1,4
polyisoprene with low or high cis-1,4polybutadiene.
However, the above mentioned polybutadiene is not always sufficient
in the hysteresis loss and fracture properties and causes a
reversion phenomenon. In addition, the use of this polybutadiene
brings about problems when blending with natural rubber or
synthetic polyisoprene. Heretofore, it has been known that the
reversion phenomenon can be solved by using polybutadiene with a
middle to high vinyl content, but this polybutadiene is still
insufficient in the hysteresis loss and fracture properties.
It is, therefore, a first object of the present invention to
provide branched styrene-butadiene copolymer rubbers having a low
hysteresis loss and improved fracture properties without causing
the reversion phenomenon.
It is a second object of the present invention to considerably
improve the rolling resistance and high-speed durability of
pneumatic tires by using the aforementioned copolymer as a base
rubber for the tread portion of a cap/base structure.
In one embodiment the present invention relates to a a branched
styrene-butadiene copolymer produced by coupling an active
styrene-butadiene copolymer anion, which is obtained by
polymerizing butadiene and styrene in the presence of ether or a
tertiary amine and an initiator of an organolithium compound in a
hydrocarbon solvent, with a tin halide compound, the improvement
wherein:
(I) a ratio of branched polymer connected with tinbutadienyl bond
in said copolymer is at least 20% by weight;
(II) a content of bound styrene in said copolymer is not less than
3% by weight but less than 25% by weight; and
(III) a content of vinyl bond in the butadiene portion of said
copolymer is not less than 30% but less than 50%.
In a second embodiment the present invention, relates to a
pneumatic tire comprising a toroidal carcass reinforcement, and a
tread portion superimposed about a crown region of said carcass and
having a cap/base structure of tread base rubber and tread cap
rubber, the improvement wherein as said tread base rubber is a
rubber composition containing at least 20 parts by weight of a
branched styrene-butadiene copolymer rubber produced by coupling a
styrene-butadiene copolymer anion with a tin halide compound, based
on 100 parts by weight of the total rubber content.
The present invention will now be described in greater detail
below.
In order to achieve the reduction of fuel consumption and the
improvement of high-speed durability in tires as described above,
it has been attempted to reduce heat build-up of the tire or
hysteresis loss of the tire, especially the tire tread portion.
One method for the reduction of hysteresis loss, is to use
materials having a low glass transition temperature such as high
cis-1,4 polybutadiene and the like or materials having a high
rebound resilience such as natural rubber and the like.
However, the use of these rubber materials extremely degrades wet
braking performance, cornering stability on wet road or wet slalom,
handling performance in high-speed running, and running stability
such as braking performance or the like, so that it is very
difficult to simultaneously accomplish these running stabilities
with the low rolling resistance and high-speed durability.
Furthermore, it is known that the low rolling resistance and the
wet skid resistance can be improved by optimizing the molecular
structure of a polymer to be used in a single tread portion as
disclosed in U.S. Pat. No. 4,334,567, but the improvement is still
not satisfactory. This is because a styrene-butadiene copolymer
rubber produced by solution polymerization using an organolithium
compound is used alone, so that the breaking strength and
elongation at breakage are low and many troubles are caused in the
running of the tire or in the manufacture of the tire. That is, in
large pneumatic tires to be mounted on truck, bus and the like,
tread damages such as rib tear (rib breaking) and the like are
produced during the running under high load and high deformation,
while when the tire after vulcanization is taken out from a mold,
molding defects are caused by damaging the tread with the pattern
of the mold. The latter case is particularly conspicuous in tires
for passenger cars having a complicated tread pattern. Therefore,
it is difficult to satisfy all performances required for the tread
portion with the single rubber composition in anyone of large tires
for truck and bus and small tires for passenger cars.
In order to overcome these drawbacks, a so-called cap/base
structure obtained by functionally separating the tread portion has
been applied to not only large tires but also low fuel consumption
tires for passenger cars as described, for example, in Japanese
Patent laid-open No. 55-99,403.
In the tread portion of such a cap/base structure, the running
stability and resistance to tread damaging are sufficiently
satisfied when the same rubber composition as used in the
conventional single tread portion is used as a tread cap rubber.
That is, a rubber composition consisting mainly of
styrene-butadiene copolymer rubber is used as a tread cap rubber in
tires for passenger cars, while natural rubber and/or polyisoprene
rubber or a rubber blend of styrene-butadiene copolymer rubber
therewith is used as a tread cap rubber in large tires for truck
and bus.
The inventors have made various studies with respect to a means for
improving the low rolling resistance and high-speed durability of
the pneumatic tire by effectively utilizing the cap/base structure
without deteriorating the running stability and resistace to tread
damaging such as rib tear or the like and found out that the low
rolling resistance and high-speed durability of the tire are
considerably improved by using a rubber composition containing
styrene-butadiene copolymer rubber of particular molecular
structure, natural rubber and polyisoprene rubber as a tread base
rubber of the tread portion facing a breaker, and as a result the
present invention has been accomplished.
In the styrene-butadiene copolymer according to the present
invention, a content of bound styrene is not less than 3% by weight
but less than 25% by weight, preferably not less than 3% by weight
but not more than 15% by weight, more particularly 5 to 15% by
weight, while a content of vinyl bond in butadiene portion is not
less than 30% but less than 50%.
When the content of bound styrene is less than 3% by weight,
fracture properties are poor, while when the content of bound
styrene is not less than 25% by weight, the hysteresis loss is
undesirably high. In the application of the copolymer to the tread
base rubber of the pneumatic tire, the content of bound styrene is
preferable to be not more than 15% by weight.
When the content of vinyl bond is less than 30%, not only the
hysteresis loss is degraded due to the formation of block
polystyrene, but also modulus and tensile properties lower in over
vulcanization when blending with natural rubber, i.e. a so-called
reversion phenomenon occurs. On the other hand, when the content of
vinyl bond is not less than 50%, not only the fracture properties
and wear resistance lower, but also the glass transition point
rises, so that the heat build-up is poor.
The copolymer according to the present invention is characterized
by containing at least 20% by weight of branched polymer connected
with tin-butadienyl bond. In case of copolymers containing branched
polymer connected with silicon-butadienyl bond or carbon-carbon
bond other than tin-butadienyl bond, the improvement of hysteresis
loss is not expected.
Furthermore, the fracture properties and hysteresis loss of the
resulting copolymer vulcanizate are improved when tin-carbon bond
in the branching of the copolymer is tin-butadienyl bond rather
than tin-styryl bond.
When the ratio of branched polymer connected with tin-butadienyl
bond is less than 20% by weight, the hysteresis loss is
substantially equal to that of the well-known low cis-1,4
polybutadiene or high cis-1,4 polybutadiene, so that the object of
the present invention cannot be achieved and also the
processability is poor.
The copolymers according to the present invention are usually
produced by polymerizing styrene and butadiene in the presence of
ether or a tertiary amine and an initiator of an organolithium
compound in a hydrocarbon solvent, adding 1 to 15 mole of
1,3-butadiene per 1 gram atom of lithium in the initiator to the
resulting polymer and then coupling it with a tin halide
compound.
The preferred copolymers according to the present invention are
obtained by polymerizing styrene and butadiene under temperature
conditions that the polymerization initiation temperature is
0.degree.-50.degree. C., the maximum access temperature is not more
than 120.degree. C. and the rising of polymerization temperature is
at least 30.degree. C. as a difference from the polymerization
initiation temperature, adding a small amount of butadiene to
change the terminal of the resulting polymer into butadienyllithium
and then coupling with a tin halide compound. In the thus obtained
copolymers, the content of vinyl bond in butadiene portion
gradually reduces toward the terminal of the polymer and the
butadiene portion at the branched connection is tin-1,4-butadienyl
bond (Sn--C--C.dbd.C--C--), so that they are low in the viscosity
during the kneading and excellent in the processability and also
the vulcanizate therefrom is excellent in the hysteresis loss.
The content of vinyl bond in the molecular chain of the
styrene-butadiene copolymer is determined by the amount of ether or
tertiary amine in the polymerization system and the polymerization
temperature. Since the amount of ether or tertiary amine in the
polymerization system is usually constant, the distribution of the
content of vinyl bond in the molecular chain of the copolymer is
changed by the hysteresis of the polymerization temperature.
Moreover, the styrene-butadiene copolymer according to the present
invention is a substantially random copolymer and is favorable that
the content of block polystyrene in bound styrene is not more than
10% as measured by a method described by I. M. Kolthoff et al in J.
Polymer Sci., Vol. 1, 429 (1946).
The Mooney viscosity of the copolymer is not particularly critical,
but its ML.sub.1+4 100.degree. C. is within a range of 20-150,
preferably 40-80.
As the hydrocarbon solvent to be used in the production of
styrene-butadiene copolymer, use may be made of hexane, heptane,
cyclohexane, benzene, xylene and mixtures thereof. The
organolithium compound includes, for example, alkyl lithiums such
as n-butyllithium, sec-butyllithium, 1,4-dilithiobutane and the
like and alkylene dilithiums, which is used in an amount of
0.02-0.2 part by weight per 100 parts by weight of the monomer.
Ether and tertiary amine are used as a randomization agent for
styrene and butadiene as well as an adjusting agent for the
microstructure of butadiene portion, a typical example of which
includes dimethoxybenzene, tetrahydrofuran, dimethoxyethane,
diethylene glycol dibutyl ether, diethylene glycol dimethyl ether,
triethylamine, pyridine, N-methylmorpholine,
N,N,N',N'-tetramethylethylene diamine, 1,2-diperidino ethane and
the like.
The coupling reaction is performed at a temperature of 50.degree.
to 120.degree. C. The tin halide compound is used in an amount of
0.2 to 3 equivalent of halogen atom per 1 equivalent of lithium
atom existent in the terminal of the polymer.
As the tin halide compound, use may be made of tin tetrachloride,
tin tetrabromide, tin methyl trichloride, tin butyl trichloride,
bis(trichlorostannyl)ethane and the like.
The styrene-butadiene copolymer according to the present invention
is used alone or in a blend with at least one rubber selected from
natural rubber, synthetic cis-1,4 polyisoprene, emulsion
polymerized styrene-butadiene copolymer, high cis-1,4
polybutadiene, low cis-1,4 polybutadiene, ethylene-propylene-diene
terpolymer and the like for use in tires as well as rubber spring,
belt, hose and other industrial goods. In this case, the copolymer
or its rubber blend is extended with oil, added with additives
usually used for vulcanizate and then vulcanized, if necessary.
When the branched styrene-butadiene copolymer according to the
present invention is used as the tread base rubber in the pneumatic
tire, it is preferable that the tread base rubber contains at least
20 parts by weight of the branched copolymer having a content of
bound styrene of not less than 3% by weight but not more than 15%
by weight and a content of vinyl bond in butadiene portion of not
less than 30% but less than 50% and not less than 30 parts by
weight of natural rubber and/or polyisoprene rubber, based on 100
parts by weight of total rubber content.
The second aspect of the present invention lies in that the rubber
composition containing the branched styrene-butadiene copolymer
connected with tin-butadienyl bond, which is obtained by coupling
styrene-butadiene copolymer anion with the tin halide compound, is
used as the tread base rubber. This results from the fact that such
a branched copolymer containing composition considerably improves
not only the heat build-up but also fracture properties,
particularly breaking strength at high temperature. In the branched
copolymer according to the present invention, the crosslink with
tin is chemically stable and is hardly subjected to oxidative
degradation or the like even at high temperature as compared with
the crosslink formed by vulcanization usually used in rubber
industry. Furthermore, since molecular chains of copolymers are
connected to each other by the coupling reaction to form a starlike
polymer, molecular weight between the crosslink points is
determined only by the molecular weight of the copolymer before the
coupling reaction and the number of copolymer molecules passing the
crosslink point is determined by the functionality of the tin
halide compound, whereby regular network is formed in the branched
copolymer. In other words, the conventional rubber composition
containing no branched copolymer has a broad distribution in the
molecular weight between crosslink points due to the presence of
only irregular crosslink points by the vulcanizing agent such as
sulfur or the like, while in the rubber composition containing the
branched copolymer the ratio of the molecular weight between
particular crosslink points becomes relatively large. These facts
are a cause of improving the fracture properties. That is, when
external force is applied to the conventional rubber composition
containing no branched copolymer, the molecular chain having a low
molecular weight between crosslink points is tensioned and strain
is concentrated thereon to selectively produce the chain breaking,
which results in the lowering of the breaking strength for the
composition. This phenomenon is less in the rubber composition
containing the branched copolymer, which results in the improvement
of the fatigue properties. This fact is considered to be a cause of
improving the fatigue properties even at high temperature because
the reinforcing action of carbon black is reduced and the crosslink
point of sulfur becomes chemically unstable at such high
temperature.
In general, when strain is applied to the rubber composition, there
are generated enthalpy stress resulting from the reinforcing action
of carbon black and entropy stress resulting from the crosslink
point. The latter is a stress generating mechanism called as a
so-called rubber elasticity and does not substantially follow the
energy loss. Therefore, in order to improve the low heat build-up
or low rolling resistance, it is sufficient to uniformize the
molecular weight between crosslink points and the number of
molecules passing the crosslink point or to reduce the number of
free terminal chains (molecular chain terminal having no crosslink
point), which can be achieved by using the branched copolymer
having a crosslinking structure as described above.
In the conventional rubber composition, the vulcanization is
usually carried out at high temperature in a short time in order to
increase mass production and productivity, during which a so-called
reversion produced by the breaking of crosslinked network becomes
conspicuous and considerably deteriorates the heat build-up of the
composition controlling the low rolling resistance and high-speed
durability. On the other hand, according to the present invention,
the deterioration of the heat build-up is improved by the use of
the rubber composition containing not less than 20 parts by weight
of the branched copolymer per 100 parts by weight of total rubber
content. When the amount of the branched copolymer is less than 20
parts by weight, the effect of suppressing the reversion cannot be
expected.
The effect of suppressing the reversion is excellent in the heat
build-up and fracture properties and is further developed when the
branched copolymer is used together with natural rubber and
polyisoprene rubber exhibiting remarkable reversion. Particularly,
the deterioration of the heat build-up due to the reversion is
considerably improved when the rubber composition for tread base
rubber contains not less than 20 parts by weight of the branched
copolymer according to the present invention and not less than 30
parts by weight of at least one of natural rubber and polyisoprene
rubber, based on 100 parts by weight of total rubber content. When
the amount of at least one of natural rubber and polyisoprene
rubber is less than 30 parts by weight, the effect of improving the
heat build-up by the synergistic action with the branched copolymer
cannot be expected.
Moreover, the rubber composition to be used as the tread base
rubber of the tire according to the present invention may contain
additives usually used in rubber industry such as vulcanization
accelerator, vulcanizing agent, supplement accelerator, carbon
black, softener, antioxidant and the like.
The rubber compositions according to the present invention as
described above are advantageously applied to anyone of tires
having a tread portion of cap/base structure. Particularly, they
are favorably used in heavy duty radial tires for truck, bus and
construction vehicles as well as radial tires for passenger
cars.
The following examples are given in illustration of the invention
and are not intended as limitations thereof.
Moreover, various measurements are made by the following
methods.
The tensile properties are measured according to the method of JIS
K-6301.
The rebound resilience at 70.degree. C. (resilience measured by
Dunlop tripsometer) is used as an index for hysteresis loss.
The wear is measured by means of Pico type abrasion machine.
The microstructure is determined by an infrared spectrophotometry
(Morero's method). Furthermore, the content of bound styrene is
determined from a calibration curve based on absorption of phenyl
group at 699 cm.sup.-1 by the infrared spectrophotometry. And also,
the ratio of branched polymer connected with tin-carbon bond is
determined by a gel permeation chromatography (GPC).
EXAMPLES 1-4, Comparative Examples 1-7
Into a reaction vessel of 5 l capacity were charged cyclohexane,
1,3-butadiene, styrene and tetrahydrofuran in predetermined amounts
as shown in the following Table 1 under nitrogen atmosphere. After
the temperature of the resulting mass was adjusted to the
predetermined initiation temperature, n-butyllithium was added to
perform polymerization while raising temperature under heat
insulation. After 40 minutes, the conversion rate for
polymerization reached to 100%.
Then, a small amount of 1,3-butadiene was added to form
butadienyllithium in the terminal of the polymer and thereafter tin
tetrachloride was added to perform coupling reaction for 30
minutes.
In Example 4, however, the coupling reaction was performed after
isothermal polymerization was carried out at 60.degree. C. for 1
hour and a small amount of 1,3-butadiene was added. In Comparative
Example 7, the coupling reaction was performed at such a state that
the terminal of the polymer is styryl anion without adding
additional 1,3-butadiene after the isothermal polymerization.
The resulting polymer solution was added with 2,6-di-tert-butyl
p-cresol, subjected to steam stripping to perform the removal of
solvent, and dried on a roll heated at 110.degree. C. to obtain a
copolymer.
Moreover, Comparative Example 5 used high cis-1,4 polybutadiene
(trade name: "BROI") made by Japan Synthetic Rubber Co., Ltd.
The copolymer was mixed with other ingredients according to the
compounding recipe shown in the following Table 2 by means of
brabender and roll and then vulcanized at 145.degree. C. for 35
minutes.
The properties of the copolymer and vulcanizate are shown in the
following Table 3.
TABLE 1
__________________________________________________________________________
Compar- Compar- Compar- Compar- Compar- Compar- ative ative ative
ative ative ative Example Example Example Example example example
example example example example 1 2 3 4 7 1 2 3 4 6
__________________________________________________________________________
Cyclohexane (g) 2,250 2,250 2,250 3,500 3,500 2,250 2,250 2,250
2,250 2,250 1,3-butadiene (g) 470 445 420 445 445 500 345 445 445
445 Styrene (g) 25 50 75 50 50 0 150 50 50 50 Tetrahydrofuran (g)
4.5 4.5 4.5 10.0 10.0 4.5 4.5 22.5 1.1 4.5 n-Butyllithium (g) 0.30
0.31 0.33 0.32 0.33 0.30 0.33 0.31 0.31 0.31 Polymerization
30.fwdarw.100 30.fwdarw.100 30-98 60 60 30.fwdarw.100 30.fwdarw.100
30.fwdarw.95 20.fwdarw.90 30.fwdarw.98 temperature (.degree.C.)*
Conversion rate (%) 100 100 100 100 100 100 100 100 100 100
Additionally added 5 5 5 5 0 0 5 5 5 5 butadiene (g) Coupling agent
SnCl.sub.4 (g) 0.15 0.15 0.15 0.15 0.16 0.15 0.15 0.15 0.15 0.035
__________________________________________________________________________
*Numerical values show polymerization initiation temperature
.fwdarw. maximum access temperature. Example 4 is an isothermal
polymerization.
TABLE 2 ______________________________________ Part by weight
______________________________________ Styrene-butadiene copolymer
100 Carbon black HAF 50 Stearic acid 1 Zinc white 3 Sulfur 1.75
Vulcanization accelerator NS* 1
______________________________________ *n-tert-butyl-2-benzothiazyl
sulfenamide
TABLE 3(a)
__________________________________________________________________________
Comparative Comparative Example 1 Example 2 Example 3 Example 4
example example
__________________________________________________________________________
1 Mooney viscosity (ML.sub.1+ 4 100.degree. C.) 54 56 55 54 55 56
Bound styrene (%) 5 10 15 10 10 0 Microstructure (%) 23/42/35
23/41/36 22/41/37 23/41/36 23/41/36 23/42/35 cis/vinyl/trans Ratio
of polymer having tin-butadienyl bond (%) 54 53 50 50 0 55 (54**)
Mooney viscosity of composition (ML.sub.1+ 4 100.degree. C.) 62 62
63 63 67 64 Properties of vulcanizate 300% modulus (kg .multidot.
f/cm.sup.2) 145 150 151 147 150 140 Tensile strength (kg .multidot.
f/cm.sup.2) 200 220 245 215 200 180 Elongation (%) 480 470 460 470
460 450 Hardness (JIS-A) 65 66 67 66 66 65 Rebound resilience
70.degree. C. (%) 74 73 72 72 69 72 Pico wear* (indicated by index)
105 105 110 105 105 100
__________________________________________________________________________
*Index value on the basis that Comparative example 1 is 100. The
larger the index value, the better the property. **Tin-styryl
bond
TABLE 3(b)
__________________________________________________________________________
Compara- Compara- Compara- Compara- Compara- tive tive tive tive
tive example 2 example 3 example 4 example 5 example
__________________________________________________________________________
6 Mooney viscosity (ML.sub.1+ 4 100.degree. C.) 53 55 54 50 55
Bound styrene (%) 30 10 10 0 10 Microstructure (%) 24/41/35 8/70/22
30/20/50 95/2/3 23/41/36 cis/vinyl/trans Ratio of polymer having
tin-butadienyl bond (%) 54 53 52 0 15 Mooney viscosity of
composition (ML.sub.1+ 4 100.degree. C.) 65 62 65 72 78 Properties
of vulcanizate 300% modulus (kg .multidot. f/cm.sup.2) 163 170 174
140 130 Tensile strength (kg .multidot. f/cm.sup.2) 255 200 220 190
180 Elongation (%) 440 370 360 630 450 Hardness (JIS-A) 70 66 66 68
66 Rebound resilience 70.degree. C. (%) 65 68 68 65 66 Pico wear*
(indicated by index) 110 90 95 105 95
__________________________________________________________________________
As apparent from Table 3, the copolymers of Examples 1-4 are
superior in the tensile strength (fracture property) to the
copolymer of Comparative Example 1 and are excellent in the
elongation as compared with Comparative Example 3. Further, they
are excellent in the rebound resilienc at 70.degree. C. as compared
with Comparative Examples 2-7.
The copolymers of Examples 1-4 are superior in the wear resistance
to those of Comparative Examples 1, 3, 4 and 6. And also, they are
low in the Mooney viscosity of their composition as compared with
that of Comparative Example 6 and are excellent in the
processability.
EXAMPLE 5
The copolymer of each of Example 1 and Comparative Example 5 was
mixed with other ingredients according to the compounding recipe
shown in the following Table 4 and then vulcanized, during which
optimum vulcanizing times at 145.degree. C. and 165.degree. C. were
determined by means of a curelastometer. Then, loss percentages in
tensile stress and tensile strength between the vulcanizate after
the optimum vulcanizing time and the vulcanizate after the
vulcanizing time corresponding to 3 times the optimum vulcanizing
time were measured as a reversion to obtain results as shown in the
following Table 5.
From the data of Table 5, it is apparent that no reversion occurs
in Example 1.
TABLE 4 ______________________________________ Part by weight
______________________________________ Natural rubber (RSS #1) 50
Copolymer of Example 1 or 50 Comparative Example 5 Carbon black HAF
50 Stearic acid 1 Zinc white 3 Sulfur 1.75 Vulcanization
accelerator 1 ______________________________________
TABLE 5 ______________________________________ Copolymer Copolymer
of of Example 1 Comparative example 5
______________________________________ Vulcanization 145 165 145
165 temperature (.degree.C.) Loss in tensile 1 3 13 24 stress (%)
Loss in tensile 1 2 16 30 strength (%)
______________________________________
EXAMPLE 7
In this example, there were prepared and used the following
copolymers A-F. The copolymers A and B corresponded to Examples 1
and 2, respectively. The copolymer C was obtained by the same
polymerization and coupling reaction as described in Example 1
except for the use of 420 g of 1,3-butadiene, 75 g of styrene, 4.0
g of tetrahydrofuran and 0.32 g of n-butyllithium. The copolymer D
corresponded to Comparative Example 1. The copolymer E was obtained
by the same polymerization as described in Example 2 without
performing the coupling reaction for butadienyllithium in the
terminal of the polymer. The copolymer F was obtained by the same
polymerization as described in Comparative Example 7.
The content of bound styrene, content of vinyl bond in butadiene
portion and ratio of polymer containing tin-butadienyl bond were
measured with respect to these copolymers to obtain results as
shown in the following Table 6.
TABLE 6 ______________________________________ Kind of copolymer A
B C D E F ______________________________________ Content of bound 5
10 15 0 10 25 styrene (wt. %) Content of vinyl 42 41 36 42 41 36
bond (wt. %) Ratio of polymer 54 53 50 55 0 0 containing tin-
butadienyl bond (wt. %) ______________________________________
According to the present invention, it is necessary that the ratio
of polymer connected with tin-butadienyl bond in the branched
copolymer is at least 20% by weight, preferably not less than 40%
by weight. In this connection, all of the copolymers according to
the present invention shown in Table 6 had the ratio of not less
than 50% by weight and exhibited a good coupling efficiency.
Then, a rubber composition containing each of the copolymers A-F
was prepared according to the compounding recipe shown in the
following Table 7 and used as a tread base rubber to manufacture a
radial tire having a tire size of 165 SR 13. The rolling resistance
and high-speed durability were measured with respect to such a tire
to obtain results as shown in the following Table 8.
Moreover, the evaluations of these properties were made as
follows:
Rolling resistance
The test tire subjected to an internal pressure of 1.7 kg/cm.sup.2
was trained on a steel drum with a diameter of 1,707.6 mm and a
width of 350 mm, which was rotated by the driving of a motor, at a
speed of 80 km/hr under JIS 100% load (385 kg) for 30 minutes and
thereafter the rotating speed of the drum was raised to 100 km/hr.
Then, the driving of the motor was stopped to run the drum by
inertia, during which the rolling resistance of the tire to the
drum was measured on a basis of deceleration speed of drum and time
change. The rolling resistance was indicated by an index according
to the following equation on the basis that the tire of Tire No. 11
is 100 (which corresponds to the rolling resistance of 5.0 kg). The
smaller the index value, the better the property. ##EQU1##
High-speed durability
The measurement was performed according to a method of FMVSS No.
109, wherein the running speed of the test tire was stepped up in
the order of 140 km/hr, 150 km/hr, 160 km/hr, 170 km/hr, 180 km/hr,
185 km/hr, 190 km/hr, 195 km/hr, 200 km/hr and 205 km/hr every 30
minutes. The high-speed durability was indicated by a step speed in
the breaking of the test tire and a lapse time at this step
speed.
TABLE 7 ______________________________________ Composition No. 1-6
7-8 9 10 11 ______________________________________ Copolymer of
Table 6 35 45 60 10 100 Natural rubber (RSS #4) 35 25 -- -- --
Polyisoprene rubber*.sup.1 -- 10 20 60 -- Polybutadiene
rubber*.sup.2 30 20 20 30 -- Carbon black HAF 40 40 40 40 60
Softener, aromatic oil 3 3 3 3 20 Antioxidant*.sup.3 1 1 1 1 1
Stearic acid 2 2 2 2 2 Zinc white 4 4 4 4 4 Sulfur 2.5 2.5 2.5 2.5
1.5 Vulcanization 1 1 1 1 1 accelerator*.sup.4
______________________________________ *.sup.1 IR2200, made by
Japan Synthetic Rubber Co., Ltd. *.sup.2 BR01, made by Japan
Synthetic Rubber Co., Ltd. *.sup.3
N--phenylN'--isopropylp-phenylenediamine *.sup.4
N--oxydiethylene2-benzothiazylsulfenamide
TABLE 8
__________________________________________________________________________
Tire No. 1 2 3 4 5 Example Example Example Comparative example
Comparative example
__________________________________________________________________________
Compounding Composi- 1 2 3 4 5 recipe for tion No. tread base Kind
of A B C D E rubber copolymer Rolling resistance 87 88 89 94 95
High-speed durability 200 .times. 27' 200 .times. 20' 200 .times.
14' 190 .times. 16' 190 .times. 9' step speed (km/hr) .times. '
time (min)
__________________________________________________________________________
Tire No. 6 7 8 9 10 11 Comparative Comparative Comparative
Comparative example Example example Example example example
__________________________________________________________________________
Compounding Composi- 6 7 8 9 10 11 recipe for tion No. tread base
Kind of F B F B B F rubber copolymer Rolling resistance 96 89 96 92
94 100 High-speed durability 195 .times. 3' 200 .times. 4' 190
.times. 28' 195 .times. 18' 195 .times. 7' 185 .times. 26' step
speed (km/hr) .times. ' time (min)
__________________________________________________________________________
Note In any one of tire Nos. 1-11, the rubber composition of
Composition No. 1 was used as a tread cap rubber for tire.
As apparent from the data of Table 8, the rolling resistance and
high-speed durability are considerably improved in the pneumatic
tire according to the present invention.
* * * * *